1. Field of the Invention
The present invention relates to a joined metal member which are joined of two metal members by liquid diffusion joining and to a method and an apparatus for fabricating the same.
2. Prior Art
There is known thermal insert technique as a conventional method for joining two metal members, which has been applied for joining, for example, valve sheets in the rim around the opening of the inlet or outlet boats shaped in the cylinder head in the field of a engine.
A proposal which is made in Japanese Patent Application JP-A 8-100701, is to braze the valve sheet to the aluminum based cylinder head body by using an Al--Zn based brazing material and a fluoride-containing flux.
Another proposal is put forward to join the two metal members by resistance welding using contact resistance heating due to contact between them, as described in Japanese Patent publication JP-A 58-13481. In this electric resistance welding an increase in calorific value on the joining surfaces between the two members has been accomplished by forming the valve sheet of a sintered material and impregnating a metal in the pores of the sheet, therefor reducing the heating value inside the body of the sheet (see Japanese Patent publication JP-A 6-58116).
Also, Another proposal has been made to form a metal film on the surface of the valve sheet and melt the film while fitting to the cylinder head body by heating (see Japanese Patent Publication JP-A 8-270499).
Further, another proposal is made, for example, as described in Japanese Patent Publication JP-A 8-200148, to join the valve sheet and cylinder head body by utilizing solid diffusion joining (e.g., pressure welding) while forming the layer plastically deformed, without forming any molten reaction layer, on the surfaces to be joined on the side of the body. Namely, this solid diffusion joining method forms a eutectic alloy by reaction between a copper coat on the valve sheet and the material of the cylinder head body, allowing the eutectic alloy to change in a liquid layer to be squeezed out of the joining surfaces.
Furthermore, as described in Japanese Patent Publication JP-A 62-199260, a method is proposed in which an alloyed layer is formed by reacting two mother materials and a brazing material which is interposed between them by heating and the rest of the non-reacting brazing material is removed from the joining surfaces thereof by pressing.
However, in the conventional methods as described above, in the thermally inserting method, both the members requires a large mass enough to prevent one member to be inserted from separating from the other and to bear the constrictive forces applied between them when they are inserted. Therefor, in the case of the cylinder head, the valve sheet is needed to increases in thickness and width, whereas it has been limited to extend the intervals of the boats and enlarge the throat diameters. Furthermore, since this method results in an insulating aerial layer interposed between the valve sheet and the cylinder head body, it is impossible to efficiently reduce the temperature in regions near the valve sheet due to lowering thermal conductivity in the regions.
The methods of joining both the members by simply brazing or resistance welding can maintain high thermal conductivity but are difficult to apply in conjunction of the valve sheet and cylinder head because of their lower joining strength. Particularly, the joining method using brazing requires the heating of the metal members in a furnace for a long period of time, which is impossible to satisfy request for the inline production. In this method the aluminum based metal member which has been previously thermally treated can often lose the thermal treatment effect during a soldering process.
On the other hand, the solid diffusion joining method above has advantages of providing much more compact valve sheets than the joining method of thermal inserting and of enabling a wide range of diversiform designs of the members. Particularly, in combination of aluminum based cylinder with the iron based valve sheets it is necessary to use the opposite procedures to suppress generation of the brittle intermetalic compounds and in the same time to diffuse iron and aluminum atoms, so that it is difficult to set the appropriate condition for joining the members and limited to improve joining strength.
In the method of removing the molten non-reacted brazing material out of the joining surfaces, after forming the alloyed layer, as mentioned above, it is possible to increase the joining strength at said surfaces because of forming each alloyed layer on each joining layer. If the brazing material has a composition far beyond the eutectic composition, in a earlier stage of pressurizing, only low melting point part (eutectic part) of the braze flow out, the high melting point part thereof left so that high thermal value is required to be supplied to perfectly melt the braze down. In this case, it takes so long to join the mother materials that the mother materials are softened in the joining surfaces, lowering the effect of destroying the oxide film on the surfaces even if they are pressed at high pressure, resulting in decreasing the effect of removal the braze out of the surface.